Fracture behavior and microstructure of as-cast NiTi shape memory alloy

Transcription

1 Trans. Nonferrous Met. Soc. China 22(2012) Fracture behavior and microstructure of as-cast NiTi shape memory alloy JIANG Shu-yong 1, ZHANG Yan-qiu 1, FAN Hong-tao 2 1. Industrial Training Centre, Harbin Engineering University, Harbin , China; 2. College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin , China Received 16 May 2011; accepted 20 October 2011 Abstract: The as-cast ingot of equiatomic nickel titanium shape memory alloy (NiTi SMA) was prepared by vacuum consumable arc melting. The tensile tests and the compressive tests with respect to as-cast NiTi SMA were performed to study its mechanical properties of fracture. The microanalysis of as-cast NiTi SMA as well as its fractured samples was performed so as to better understand microstructure evolution and fracture behavior of NiTi SMA. Under tensile loading, the as-cast NiTi SMA shows higher plasticity and is characterized by ductile fracture at 750 C, but it demonstrates poorer plasticity and is characterized by cleavage fracture as well as transcrystalline fracture at room temperature and 100 C. Under compressive loading at 100 C, the as-cast NiTi SMA is characterized by shear fracture where the normal to the shearing fracture surface inclines about 45 to the compressive axis, and belongs to cleavage fracture where the cracks expand via transcrystalline fracture. Key words: NiTi alloy; shape memory alloy; as-cast NiTi; microstructural evolution; fracture behavior 1 Introduction Nickel titanium shape memory alloy (NiTi SMA) is widely used in engineering fields because it possesses shape memory effect and superelasticity, which are attributed to thermo-elastic martensite phase transformation of NiTi SMA [1]. The application of shape memory effect and superelasticity of NiTi SMA to engineering fields is considerably dependent on the phase transformation temperature. It is well known that NiTi SMA shows excellent superelasticity only above the austenite transformation finish temperature A f, and exhibits full shape recovery when heated to the temperature above A f. The transformation temperatures of NiTi SMA are extremely sensitive to a small variation in the Ni or Ti content. As for NiTi SMA having greater than 55.0% Ni (mass fraction), a 1% deviation in Ni (or Ti) content will result in approximately a shift of 100 C in transformation temperatures [2]. FRENZEL et al [3] demonstrated that the increase of Ni content in NiTi SMA decreases the phase transformation temperatures as well as the width of thermal hysteresis and the heat of transformation. Therefore, the strict requirement on any melting by tightly controlling the ratio of Ni to Ti so as to meet the required tolerance in the transformation temperatures plays a significant role in successful application of NiTi SMA to engineering fields [4]. The processes to melt NiTi SMA are commonly made up of vacuum induction melting (VIM) and vacuum arc melting (VAM) which includes consumable arc melting and non-consumable arc melting [5,6]. VIM contributes to homogeneity of chemical composition of NiTi SMA, but it needs using graphite crucible which leads to the reaction between Ti and C. As a result, titanium carbide (TiC) is formed in NiTi SMA during melting and solidification, which results in the increase of Ni content of the matrix of the alloy and thus decreases the martensite transformation temperature [7,8]. VAM needs no graphite crucible and thus causes little contamination of NiTi SMA from the crucible, but it usually leads to less homogeneous chemical composition of NiTi SMA ingot and requires multiple re-melting [9]. The inhomogeneous chemical composition of as-cast NiTi SMA ingot inevitably leads to uneven microstructure and thus has an influence on the mechanical properties of NiTi SMA. Cold working and heat treatment play important roles in guaranteeing the final mechanical properties of NiTi SMA [10,11]. Foundation item: Project ( ) supported by the National Natural Science Foundation of China; Project (HEUCFR1132) supported by the Fundamental Research Funds for the Central Universities of China Corresponding author: JIANG Shu-yong; Tel: ; jiangshy@sina.com DOI: /S (11)

2 1402 JIANG Shu-yong, et al/trans. Nonferrous Met. Soc. China 22(2012) Some researchers investigated the microstructures and the mechanical properties of as-cast NiTi SMA, which lays the foundations for the subsequent thermal processing of as-cast NiTi SMA. KHAMEI and DEHGHANI [12] established the constitutive equation of as-cast NiTi SMA with homogenized annealing at a high temperature which complies with the Zener- Hollomon parameter and the fact is obtained that NiTi SMA exhibits obviously dynamic recrystallization under quasi-static compression deformation at the temperatures ranging from 950 to 1050 C [13]. MORAKABATI et al [14] studied the hot tensile properties of as-cast NiTi SMA at a strain rate of 0.1 s 1 at the elevated temperatures and found that as-cast NiTi SMA shows a maximum hot ductility at the temperatures ranging from 750 to 1000 C and dynamic recrystallization at the temperatures ranging from 800 to 1000 C, where the tensile fractograph of NiTi SMA belongs to a typical ductile rupture. RAGHAVENDRA et al [15] investigated the tensile fracture of the annealed NiTi SMA under quasi-static and dynamic loading at various temperatures and discovered that NiTi SMA shows quasi-cleavage fracture at a low temperature of 196 C as well as the typical ductile fracture at high temperatures ( C), and the mixture of the two at room temperature [15]. This study is aimed at understanding fracture and microstructure of as-cast NiTi SMA which experiences no heat treatment or plastic working history. So far, fracture behaviour and microstructure of as-cast NiTi SMA have been reported under tensile and compressive loading in the few literatures. 2 Experimental were carried out on INSTRON 5500R at the strain rate of s in order to obtain the mechanical properties of as-cast NiTi SMA. Metallographic investigation of as-cast NiTi SMA as well as the NiTi SMA samples after the tensile tests was performed. Specimens for microstructure observation were cut from as-cast NiTi SMA and the NiTi SMA samples after the tensile and compressive tests. All specimens were etched in a solution containing HF, HNO 3 and H 2 O whose volume ratio was 1:4:5 and then studied by means of OLYMPUS metallographic microscope. The fracture surface of the tensile samples was observed by QUANTA200 scanning electron microscope (SEM). Fig. 1 Photo of as-cast ingot of NiTi SMA The ingot of NiTi SMA with nominal compositions of 50.0%Ni 50.0%Ti (mole fraction) was prepared in a water-cooled copper crucible by means of vacuum consumable arc melting, where the raw materials used were the Ni powder with the purity of 99.97% and the sponge Ti with the purity of 99.95%. The Ni powder and sponge Ti were mixed according to the equiatomic proportion, and then were made into the electrode bulk by means of sintering at high temperature and high pressure in the vacuum atmosphere. The electrode bulk was melted by means of vacuum consumable arc melting and then was cast into the ingot of NiTi SMA with the diameter of 80 mm and the height of 80 mm, as shown in Fig. 1. The tensile samples and the compressive samples were prepared from as-cast ingot of NiTi SMA by means of machining. The dimensions of the tensile samples are shown in Fig. 2. The compressive samples possess the diameter of 6 mm and the height of 9 mm. The tensile tests and compressive tests with respect to the samples Fig. 2 Schematic diagram of dimensions of NiTi SMA tensile sample (unit: mm) 3 Results and discussion 3.1 Microstructure of as-cast NiTi SMA Figure 3 shows the microstructures of as-cast NiTi SMA, from which it can be seen that the microstructure of the as-cast NiTi SMA which consists of dendritic grains and equiaxed grains is very inhomogeneous. Figure 4 shows the binary phase diagram of the NiTi alloy [1]. NiTi SMA with equiatomic ratio belongs to β parent phase at the high temperature and possesses BCC structure. When cooling to 1090 C, the structure of the parent phase of TiNi alloy is transited to austenite with B2 type ordered structure. With the continuous decrease of temperature, the B2 austenite is transformed into the

3 JIANG Shu-yong, et al/trans. Nonferrous Met. Soc. China 22(2012) B19 martensite. The space group of the B19 martensite is P2 1 /m with a monoclinic unit cell. Therefore, theoretically, the microstructure of as-cast NiTi SMA at room temperature should be made up of B2 austenite or B19 martensite, or a mixture of the two ones. However, in fact, the as-cast NiTi SMA consists of B19 martensite, B2 austenite and Ti 2 Ni phase simultaneously at room temperature, which will be reported in another literature. The phenomenon is attributed to the segregation of as-cast NiTi SMA during crystallization. Fig. 3 Optical microstructure of as-cast NiTi SMA ingot 3.2 Tensile fracture of as-cast NiTi SMA Tensile fracture at 750 C Figure 5 indicates the fractograph of as-cast NiTi SMA after tensile fracture at 750 C. It can be obviously seen from Fig. 5 that NiTi SMA shows a great macroscopic plastic deformation before tensile fracture. The fractograph looks like blade and is characterized by orientation, which belongs to the typical ductile rupture. In order to better understand the deformation law of NiTi SMA under tensile loading at high temperature, the corresponding tensile fractured sample in Fig. 5 is divided into three zones including zone A, zone B and zone C against the direction perpendicular to the blade along the longitudinal cross-section, which is shown in Fig. 6. The corresponding microstructures of NiTi SMA in the three zones are illustrated in Fig. 7. Zone A belongs to the severely plastic deformed one in which the grains of NiTi SMA are considerably elongated along the tensile direction, and thus shows the obvious feature of fibrous tissue. Zone C belongs to the slightly plastic deformed one in which no obvious plastic deformations takes place and the microstructure shows a characteristic of the equiaxed grains. Zone B belongs to the transient one between zone A and zone B, where a certain plastic deformation occurs. Fig. 5 Fractograph of NiTi SMA after tensile fracture at 750 C: (a) Fractured sample; (b) SEM image Fig. 6 Schematic diagram of different zones at tensile fractured sample of NiTi SMA at 750 C Fig. 4 Phase diagram of NiTi alloy: (a) Metastable Ti 3 Ni 4 phase; (b) Phase equilibrium between TiNi and Ti 3 Ni 4 phases added in dashed zone of (a) Tensile fracture at room temperature Figure 8 demonstrates the fractograph of NiTi SMA after tensile fracture at room temperature. The slight plastic deformation of NiTi SMA occurs before tensile fracture and the fractograph of NiTi SMA shows the

4 1404 JIANG Shu-yong, et al/trans. Nonferrous Met. Soc. China 22(2012) brittle fracture feature. Furthermore, the fractograph consists of cleavage plane and tear ridge around the cleavage plane. Therefore, the fractograph of NiTi SMA is typical of cleavage fracture. It can be obviously seen from the microstructure of NiTi SMA sample after tensile fracture in Fig. 9 that the crack of NiTi SMA expands by means of transcrystalline fracture. Fig. 9 Microstructure of tensile fractured sample of NiTi SMA at room temperature Fig. 7 Microstructures of different zones in tensile fractured sample of NiTi SMA at 750 C: (a) Zone A in Fig. 6; (b) Zone B in Fig. 6; (c) Zone C in Fig Tensile fracture at 100 C Figure 10 demonstrates the fractograph of NiTi SMA sample after tensile fracture at 100 C. The fractograph of NiTi SMA is also characterized by the typical cleavage fracture, but is very different from that at room temperature. NiTi SMA belongs to the full martensite structure at 100 C which is far less than martensite transformation finish temperature (M f ), so the fractograph of NiTi SMA at 100 C possesses a characteristic of fracture surface of martensite. However, the microstructure of NiTi SMA is a mixture of austenite and martensite at room temperature, so the fractograph is a product of fracture surface of austenite and martensite. Furthermore, the fractograph of NiTi SMA in Fig. 10 may be attributed to the fracture of microstructure of dendritic grains. Figure 11 shows the microstructure of NiTi SMA sample after tensile fracture at 100 C, Fig. 8 Fractograph of NiTi SMA after tensile fracture at room temperature: (a) Fractured sample; (b) SEM image Fig. 10 Fractograph of NiTi SMA after tensile fracture at 100 C: (a) Fractured sample; (b) SEM image

5 JIANG Shu-yong, et al/trans. Nonferrous Met. Soc. China 22(2012) Fig. 11 Microstructure of tensile fractured sample of NiTi SMA at 100 C where the crack still expands by means of transcrystalline fracture. 3.3 Compressive fracture of as-cast NiTi SMA The compressive deformation of as-cast NiTi SMA samples at 100 C leads to fracture of the compressed sample at the strain rate of s 1, as shown in Fig. 12. It can be seen from Fig. 12 that the fractured sample is characterized by shear fracture and the angle between the normal to the shearing fracture surface and the compressive axis is about 45. The fractograph of the corresponding fractured sample shown in Fig. 12 has a characteristic of river pattern containing cleavage plane Fig. 13 Microstructure of compressed samples of as-cast NiTi SMA at 100 C and tear ridge and thus belongs to cleavage fracture. Furthermore, the microstructure of the compressive fractured sample of as-cast NiTi SMA at 100 C in Fig. 13 indicates that the cracks expand by means of transcrystalline fracture. 4 Conclusions 1) The as-cast NiTi SMA prepared by vacuum consumable arc melting shows inhomogeneous microstructure, which consists of dendritic grains and equiaxed grains, and is a mixture of B19 martensite, B2 austenite and Ti 2 Ni phase at room temperature. Therefore, multiple remelting is an approach to obtain NiTi SMA with uniform microstructure. 2) Under tensile loading, the as-cast NiTi SMA is characterized by ductile fracture at 750 C, but cleavage fracture at room temperature as well as 100 C. Furthermore, the crack expands by means of transcrytalline fracture during fracture of as-cast NiTi SMA at room temperature as well as 100 C. 3) Under compressive loading at 100 C, the fractured sample of the as-cast NiTi SMA is characterized by shear fracture and the angle between the normal to the shearing fracture surface and the compressive axis is about 45. The fractograph of the as-cast NiTi SMA has a characteristic of river pattern containing cleavage plane and tear ridge, and thus belongs to cleavage fracture. The cracks expand by means of transcrystalline fracture. References Fig. 12 Fractographs of fractured samples of as-cast NiTi SMA under compression at 100 C: (a) Fractured sample; (b) SEM image [1] OTUKA K, REN X. Physical metallurgy of Ti Ni-based shape memory alloys [J]. Progress in Materials Science, 2005, 50(5): [2] WU M H. Fabrication of nitinol materials and components [J]. Material Science Forum, 2001, : [3] FRENZEL J, GEORGE E P, DLOUHY A, SOMSEN C, WAGNER

6 1406 JIANG Shu-yong, et al/trans. Nonferrous Met. Soc. China 22(2012) M F X, EGGELER G. Influence of Ni on martensitic phase transformation in NiTi shape memory alloys [J]. Acta Materialia, 2010, 58(9): [4] MEHRABI K, BAHMANPOUR H, SHOKUHFAR A, KNEISSL A. Influence of chemical composition and manufacturing conditions on properties of NiTi shape memory alloys [J]. Materials Science and Engineering A, 2008, : [5] FRENZEL J, ZHANG Z, NEUKING K, EGGELER G. High quality vacuum induction melting of small quantities of NiTi shape memory alloys in graphite crucibles [J]. Journal of Alloys and Compounds, 2004, 385(1 2): [6] NAYAN N, GOVIND, SAIKRISHNA C N, RAMAIAH V K, BHAUMIK S K, NAIR S K, MITTAL M C. Vacuum induction melting of NiTi shape memory alloys in graphite crucible [J]. Materials Science and Engineering A, 2007, 465(1 2): [7] ZHANG Z H, FRENZEL J, NEUKING K, NEUKING K, EGGELER G. On the reaction between NiTi melts and crucible graphite during vacuum induction melting of NiTi shape memory alloys [J]. Acta Materialia, 2005, 53(14): [8] ZHANG Z H, FRENZEL J, SOMSEN C, PESICKA J, NEUKING K, EGGELER G. Orientation relationship between TiC carbides and B2 phase in as-cast and heat-treated NiTi shape memory alloys [J]. Materials Science and Engineering A, 2006, : [9] RIGO O D, OTUBO J, MOURA NETO C, MEI P R. NiTi SMA production using ceramic filter during pouring the melt [J]. Journal of Materials Processing Technology, 2005, : [10] MOHAMED E M, MAHMOUD F. Effect of cold work and annealing on the structure and characteristics of NiTi alloy [J]. Materials Science and Engineering A, 2009, 519(1 2): [11] MILLER D A, LAGOUDAS D C. Influence of cold work and heat treatment on the shape memory effect and plastic strain development of NiTi [J]. Materials Science and Engineering A, 2001, 308(1 2): [12] KHAMEI A A, DEHGHANI K. Modeling the hot-deformation behavior of Ni 60wt% -Ti 40wt% intermetallic alloy [J]. Journal of Alloys and Compounds, 2010, 490(1 2): [13] DEHGHANI K, KHAMEI A A. Hot deformation behavior of 60Nitinol (Ni 60wt% Ti 40wt% ) alloy: Experimental and computational studies [J]. Materials Science and Engineering A, 2010, 527(3): [14] MORAKABATI M, ABOUTALEBI M, KHEIRANDISH S, KARIMI TAHERI A, ABBASI S M. Hot tensile properties and microstructural evolution of as cast NiTi and NiTiCu shape memory alloys [J]. Materials and Design, 2011, 32(1): [15] RAGNAVENDRA R A, JIANG F C, BINGERT J F, VECCHIO K S. Influence of cold work and texture on the high-strain-rate response of Nitinol [J]. Materials Science and Engineering A, 2010, 527(20): 铸态镍钛形状记忆合金的断裂行为及显微组织 江树勇 1, 张艳秋 1, 范红涛 2 1. 哈尔滨工程大学工程训练中心, 哈尔滨 ; 2. 哈尔滨工程大学材料科学与化学工程学院, 哈尔滨 摘要 : 通过真空自耗电极熔炼法制备等原子比镍钛形状记忆合金 为了研究其断裂力学性能, 进行铸态镍钛形状记忆合金的拉伸和压缩实验 为了更好地理解镍钛形状记忆合金的组织演变及断裂行为, 分析铸态镍钛形状记忆合金及其断裂样品的显微组织 在拉伸加载下, 镍钛形状记忆合金在 750 C 时具有较高的塑性, 表现为韧性断裂, 但在室温和 100 C 时表现出较差的塑性, 具有解理断裂和穿晶断裂的特征 在 100 C 的压缩加载下, 铸态镍钛形状记忆合金发生剪切断裂, 剪切断裂面法线与压缩轴呈 45, 具有解理断裂的特征, 裂纹经由穿晶断裂而扩展 关键词 : 镍钛合金 ; 形状记忆合金 ; 铸态镍钛 ; 组织演变 ; 断裂行为 (Edited by LI Xiang-qun)